Nostrils smell differently 1

By Karl S. Kruszelnicki

We have two eyes on the front of our heads. Each eye gets a slightly different picture of the world, and this stereo image gives us the impression of depth. Our two ears pick up sounds at slightly different times, and this lets us work out where the noise is coming from. And it now turns out, that each of our two nostrils can pick up different smells.

This research was done by Noam Sobel and colleagues from Stanford University in California, and the Ministry of Environmental Protection in Israel. They looked at how different odours stimulate your smelling apparatus. You do most of your "smelling" with a part of your nose called the "olfactory epithelium". It's a patch of yellowish tissue high up in the roof of your nose. Normally, it does not have a very good air flow running over it, but this changes when you sniff deeply.

In this yellowish patch, there are sensory cells specially adapted for smelling, as well as supporting cells to hold the whole structure together. There are also gland cells that supply a wet layer of mucus to cover the yellow patch. Different chemicals in the air enter your nose, dissolve in this wet layer, and then excite the sensory cells.

Your human yellow patch of olfactory epithelium has about 40,000 sensory cells in each square millimetre, with a total area of around 250 square millimetres (about the size of your thumbnail) - this gives a total of about 10 million cells that can detect odours. A rabbit on the other hand has around 120,000 sensory cells per square millimetre giving a total of 100 million sensory cells - which put together, have a total surface area greater than the skin of the rabbit's body!

Now it turns out that you do not breath equally through each nostril. This was first noted by Kayser, a German nose specialist, or rhinologist, way back in 1895 - and it's still a little bit controversial today. You see, we all have three sets of erectile tissue in each nostril - the same sort of erectile tissue found in the penis and clitoris.

It seems that in some people, the erectile tissue in one nostril will swell, while at the same time, in the other nostril, it will shrink. So while one nostril is passing a lot of air, the other isn't. And then a little bit later it swaps over. The complete Nasal Cycle takes anything from 40 minutes to several hours to run. Only a small number of people have the classical "one nostril with good air flow while the other one has bad air flow", with a regular rhythmical changeover. But most of us have some degree of rhythmical change of air flow from one nostril to the other. This is called the Nasal Cycle. It seems that this Nasal Cycle gets weaker as you get older.

Also, if you lay down on one side, then after about 12 minutes, the erectile tissue in the nostril on that side will begin to engorge and swell. This might be due to sensors in your chest and pelvis.

Now if you look at the odour chemicals that land on your olfactory epithelium, you can break them down into two types - the ones that dissolve quickly, and the ones that dissolve slowly.

The ones that dissolve slowly have their maximum effect in the nostril which has a slow movement of air. This gives time for the slow-dissolving chemicals to absorb into the mucus covering the olfactory epithelium. But if the air is moving fast, they get whisked over the olfactory epithelium before they get a chance to dissolve.

The story is completely reversed for the chemicals that dissolve quickly. They have their strongest effect when they're in an air stream that's moving quickly. In this case, they get to land on a large area of olfactory epithelium, and stimulate your brain more. But when they're in an air stream that's moving slowly, they all dissolve immediately in just a little patch of the olfactory epithelium.

Now we're not sure exactly what advantage there is in having the airflow different in each nostril. But the authors of this paper offer their theory. If, thanks to the Nasal Cycle, one nostril is passing lots of air and the other is passing less air, then the same chemicals drawn into each nostril will give different responses in each nostril. This "stereo" smelling ability will let you interpret the outside world better.